A solar cell is a semiconductor device that converts photons from the sun (solar light) into electricity. Fundamentally, the solar cell needs to photogenerate charge carriers (electrons and holes) in a light-absorbing material, and separate the charge carriers to electrically conductive contacts that will transmit the electricity.
First generation photovoltaics comprise large-area single layer p-n junction diodes, which generate usable electrical energy from light sources with the wavelengths of solar light. These cells are typically made using silicon. Second generation photovoltaic devices are based on multiple layers of p-n junction diodes. Each layer is designed to absorb a successively longer wavelength of light (lower energy), thus absorbing more of the solar spectrum and increasing the conversion efficiency and thus the amount of energy produced. The third generation of photovoltaics is quite different from the first two generations, and is broadly defined as a semiconductor device which does not rely on a traditional p-n junction to separate photogenerated charge carriers. These new devices include dye sensitized cells, organic polymer cells, and quantum dot solar cells.
All solar cells require a light absorbing material contained within the cell structure to absorb photons and generate hole electron pairs via the photovoltaic effect. The materials used in solar cells tend to have the property of preferentially absorbing the wavelengths of solar light that reach the earth surface. One second generation solar cell embodiment comprises CIGS-based solar cells. CIGS are multi-layered thin-film composite solar cells. The abbreviation CIGS stands for copper indium gallium selenide (CuIn1−xGaxSe2). Unlike the basic silicon solar cell, which can be accurately modeled as a simple p-n junction, CIGS based solar cells are best described by a more complex heterojunction model. Solar cells based on CIGS have achieved the highest efficiency of all thin film solar cells.
Solar cells based on p-type CIGS absorbers have been fabricated on glass, polymer or stainless steel substrates using various deposition techniques. A cross sectional view of a typical CIGS device 100 is shown in FIG. 1. Incident sunlight 102 is partially blocked by the metallic grid shown as Ni/Al fingers 105, which covers approximately 5% of the surface of the device, and is partially reflected by the surface of the transparent conducting-oxide (TCO) layer, shown as a ZnO/ZnO:Al layer 110 due to the difference in the index of refraction. Some short-wavelength photons are absorbed in the n-CdS layer 125. Most of the sunlight, however, enters the semiconductor and is absorbed in the CIGS absorber layer 130. CIGS absorber layer 130 is shown disposed on molybdenum layer 135, which is disposed on the soda lime substrate 140 shown.
The front metal contact fingers (Ni/Al) 105 are not critical to the photovoltaic operation. The ZnO 110 and CdS layers 125 are usually n-type, and the CIGS layer 130 is usually p-type. The semiconducting junction is formed at or near the CdS 125-CIGS 130 (n-p) interface. Electrons that are generated within the junction-field region or within about one diffusion length of the n-p junction will generally be collected.
CuIn1−xGaxSe2−ySy (CIGSS) is a sulfur comprising variant of CIGS which as noted above is based on CuIn1−xGaxSe2. CIGSS is by far the most promising material for thin film photovoltaic devices. There is constant research performed to increase photovoltaic conversion efficiency. What is needed is a relatively simple process and/or material or structural change(s) to provide a significant increase in photovoltaic conversion efficiency.